1. Field of the Invention
The invention relates to nanometer scale material, and more particularly to a method for forming nanometer scale dot-shaped material.
2. Description of the Related Art
Flexible electronic technology offers product design flexibility, light weight, low cost products and quick fabrication processes. The substrates and materials used for the flexible electronic technology need to be flexible. Accordingly, flexible electronic materials need to be selected from materials that can be printed and fabricated at low temperatures.
Generally, conventional materials used for printing are conductive metal ink, which is a conductive coating formed by high temperature sintering. Conventional conductive metal ink is usually used for rigid ceramic substrates. The conventional conductive metal ink comprises an organic polymer, conductive metal particles glass and other additions, such as a dispersion agent or a rheology agent. After a degreasing process at a high temperature of 250-450° C. and a co-sintering process at a temperature of 850-1100° C., the organic polymer is removed and the conductive metal particles are transformed into a metal crystal having a continuous phase. The adhesion strength of the conventional conductive coating is provided by the glass softening. However, the conventional metal ink is only applicable for rigid substrates formed by high temperatures.
Generally, conventional flexible electronic materials are low temperature cured epoxy resin type conductive paste, which is made from polymer, used as a major material, having sheet-shaped metal conductive particles therein. Although the conventional conductive epoxy resin can be formed at a temperature lower than 200° C., the conductivity thereof is poor and the conventional conductive epoxy resin can not be soldered. Additionally, the adhesion strength and the conductivity of the conventional conductive epoxy resin, respectfully, decreases due to subsequent processes.
Currently, the method used to enhance the conductivity of a conductive metal ink, is to coat another metal on sub-micrometer scale particles. U.S. Pat. No. 5,139,890 discloses coating a thick silver film on a surface of copper or copper alloy particles and then coating a thin gold film on the silver film. The silver film needs to have a thickness of at least 3.5 μm, and the gold film on the silver film needs to have a thickness smaller than 0.5 μm. The silver film and the gold film both are formed by an electroplating, an electrolytic deposition, an electroless plating or a vacuum vapor deposition process.
Japan Patent No. 2005-267900 discloses a low temperature conductive ink technology. Specifically, a mixture utilizing silver powders mixed with nanometer scale silver oxide is formed and dispersed in an ink. Then, an organic compound with reducing ability is added in the ink to form a high conductive ink which can be cured by a low temperature sintering process. The silver oxide has an average diameter of 5 nm-15 μm. The silver oxide is formed by salting of the silver nitrate, adding a polymer dispersant and a surfactant thereto, dissolving the resulting mixture in water, and adding a water-based oxidant to form a silver oxide precipitate. The silver particles used in the ink have a diameter of about 20 nm-15 μm. The shape of the silver particle may be sphere shaped or a sheet. The ink can be used in a coating process such as a screen printing, a plate printing or a flexible plate printing coating process and can be sintered at a thermal treatment temperature of 200° C.
In addition, K. S. Park et al. disclosed in “surface modification by silver coating for improving electrochemical properties of LiFePO4” in the Solid State Communications, Volume 129, Issue 5, February 2004, Pages 311-314, using silver ions, as a coating on a surface of a positive electrode of a lithium battery made of lithium-iron-phosphorus compound oxide. Specifically, a silver nitrate solution was coated on a surface of a positive electrode by an electrochemical and wetting mixed coating method, such that the capacitance of the lithium battery was enhanced to 140 mAh/g and an initial voltage thereof was reduced to 3.3V.